(a) Technical Field
The present invention relates to a hydrophobic porous medium and a method of manufacturing the same. More particularly, it relates to a highly hydrophobic porous medium and a method of manufacturing the same.
(b) Background Art
An electrochemical reaction in a polymer electrolyte membrane fuel cell (PEMFC) for electricity generation is as follows. Hydrogen supplied to an anode (“oxidation electrode”) in a membrane electrode assembly (MEA) of the fuel cell is dissociated into hydrogen ions (protons, H+) and electrons (e−). The hydrogen ions are transmitted to a cathode (“reduction electrode”) through a polymer electrolyte membrane, and the electrons are transmitted to the cathode through an external circuit such that electricity is generated by the flow of electrons.
Moreover, at the cathode, the oxygen molecules, protons, and electrons react with each other to produce electricity and heat, and at the same time, produce water as a reaction by-product.
The area expressing the electrochemical performance of the fuel cell is generally classified into three regions: (i) an “activation loss” region due to loss of electrochemical reaction kinetics; (ii) an “ohmic loss” region due to contact resistance at interfaces between respective components and loss of ionic conduction in the polymer electrolyte membrane; and (iii) a “mass transport loss” or “concentration loss” region due to the limitations of mass transport of reactant gases [R. O Hayre, S. Cha, W. Colella, F. B. Prinz, Fuel Cell Fundamentals, Ch. 1, John Wiley & Sons, New York (2006)].
When an appropriate amount of water produced during the electrochemical reaction is present, it preferably serves to maintain the humidity of the polymer electrolyte membrane. However, when an excessive amount of water produced is not appropriately removed, “flooding” occurs at high current density, preventing the reactant gases from being efficiently supplied to the fuel cell and thereby increasing voltage loss [M. M. Saleh, T. Okajima, M. Hayase, F. Kitamura, T. Ohsaka, J. Power Sources, 167, 503 (2007)].
A typical porous medium that constitutes the fuel cell is a gas diffusion layer (GDL), which has a structure in which a microporous layer (MPL) and a macroporous substrate or backing are combined together.
Commercially available gas diffusion layers have a duel layer structure including a microporous layer having a pore size of less than 1 micrometer when measured by mercury intrusion and a macroporous substrate or backing having a pore size of 1 to 300 micrometers [X. L. Wang, H. M. Zhang, J. L. Zhang, H. F. Xu, Z. Q. Tian, J. Chen, H. X. Zhong, Y. M. Liang, and B. L. Yi, Electrochimica Acta, 51, 4909 (2006)].
The gas diffusion layer is attached to the outer surface of catalyst layers for the anode and cathode coated on both surfaces of the polymer electrolyte membrane in the fuel cell. The gas diffusion layer functions to supply reactant gases such as hydrogen and air, transmit electrons produced by the electrochemical reaction, and discharge water produced by the reaction to minimize the flooding phenomenon in the fuel cell [L. Cindrella, A. M. Kannan, J. F. Lin, K. Saminathan, Y. Ho, C. W. Lin, J. Wertz, J. Power Sources, 194, 146 (2009); X. L. Wang, H. M. Zhang, J. L. Zhang, H. F. Xu, Z. Q. Tian, J. Chen, H. X. Zhong, Y. M. Liang, B. L. Yi, Electrochim. Acta, 51, 4909 (2006)].
Especially, in order to increase the mass transport and maintain high cell performance by effectively removing the water produced during the electrochemical reaction of the fuel cell, it is very important to impart hydrophobicity to the microporous layer and the macroporous substrate by appropriately introducing a hydrophobic agent such as polytetrafluoroethylene (PTFE) into them [S. Park, J.-W. Lee, B. N. Popov, J. Power Sources, 177, 457 (2008); G.-G. Park. Y. J.-Sohn, T. H. Yang, Y.-G. Yoon, W.-Y. Lee, C. S. Kim, J. Power Sources, 131, 182(2004)].
However, a wet chemical process has conventionally been used to impart hydrophobicity, and thus the manufacturing process itself is complicated and it is difficult to uniformly distribute the hydrophobic agent such as PTTE on the gas diffusion layer.
Moreover, according to the conventional process for manufacturing the gas diffusion layer, it is difficult to further impart high hydrophobicity or super-hydrophobicity corresponding to a contact angle (static constant angle) of 150° or more to a porous medium which have already been subjected to waterproof treatment.
In conventional studies, there have been various attempts to impart hydrophilicity to the surface of the porous medium using various plasma processes such as oxygen, nitrogen, ammonia, silane (SiH4), organometallics, etc. [D. R. Mekala, D. W. Stegink, M. M. David, J. W. Frisk, US 2005/0064275 A1 (2005); Korean Patent Publication No. 10-2006-0090668 (2006)], which, however, are different from the object of the present invention to impart high hydrophobicity to the porous medium.
In addition, there have been attempts to employ plasma surface treatment techniques during manufacturing of the electrodes of the MEA [G. H. Nam, S. I. Han, Korean Patent Publication No. 10-2009-0055301 (2009); M. G. Min, G. S. Chae, S. G. Kang, Korean Patent No. 10-0839372 (2008); W. M. Lee, 1, G. Goo, J. H. Sim, Korean Patent No. 10-0681169 (2007); H. T. Kim, H. J. Kwon, Korean Patent No. 10-0599799 (2006)], which, however, relate to a process for forming a catalyst layer comprising a catalyst and a binder. That is, these methods are to chemically form a hydrophilic or hydrophobic surface by modifying the surface of the catalyst layer using plasma techniques, and with these methods, there are limitations in forming high hydrophobicity on the surface of the porous medium.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.